Process for fabry-perot filter train configuration using derived mode field size

ABSTRACT

A process for configuring a tunable MOEMS filter train comprises determining a spectral response of a MOEMS tunable filter. A spectral separation between different order modes, or free spectral range, is then determined for the filter. This information is then related to a mode size of a desired mode of the tunable filter. With this information, lenses for the optical train are provisioned, and then installed so that light is launched into the optical filter at the desired mode size to thereby maximize the SMSR of the filter train.

BACKGROUND OF THE INVENTION

[0001] Tunable optical filters are useful in situations requiringspectral analysis of an optical signal. They can also be used, however,as intra-cavity laser tuning elements or in tunable detectors, forexample. One of the most common, modem applications for these devices isin wavelength division multiplexing (WDM) systems. WDM systems transmitmultiple spectrally separated channels through a common optical fiber.This yields concomitant increases the data throughput that can beobtained from a single optical fiber. There are additional advantagesassociated with the ability to use a single fiber amplifier to amplifyall of the channels on an optical link and its use as a platform fordynamic channel/wavelength routing.

[0002] Tunable filters that operate in these WDM systems must typicallybe high quality/high finesse devices. Currently proposed standardssuggest channel spacings of 100 GigaHertz (GHz) to channel spacings astight as 50 GHz in the ITU grid; some systems in development havespacing of 20 GHz and less. Tunable filter systems that operate insystems having such tight channel spacings must have correspondinglysmall passbands when operating as monitors, receivers, and routingdevices.

[0003] Typically, the design of the tunable filters is based on a classof devices generally referred to as Fabry-Perot (FP) etalons. Thesedevices have at least two highly reflective elements defining theFabry-Perot cavity. The tunability functionality is provided bymodulating the optical length of the cavity.

[0004] Since these tunable filters are typically incorporated intolarger systems offering higher levels of functionality and because theFabry-Perot cavity must be modulated over distances corresponding to thewavelength of light that it is filtering, typically around 1,000 to2,000 nanometers (nm) in wavelength, microoptical electromechanicalsystems (MOEMS) technology is typically used to fabricate the tunablefilters. The most common implementation pairs anelectrostatically-deflectable reflective optical membrane with a fixedreflector. Thin film technology is typically used to obtain thereflectivity. High finesse systems can require dielectric mirrors havinggreater than seven layers.

[0005] A common metric for characterizing the quality of tunable filtersystems is the side mode suppression ratio (SMSR). This is the ratiobetween the magnitude of the lowest order mode in the spectral plot ofthe filter's characteristic and the magnitude of the next largest mode,which is typically the next higher order mode.

[0006] A general configuration for MOEMS tunable filter Fabry-Perotcavities is termed a curved-flat cavity. In such cavities, one of thereflectors is near planar and the other reflector is curved. If thecurved reflector has a spherical profile, the cavity is sometimesreferred to as a hemispherical cavity.

[0007] When hemispheric tunable filters are used, for example, theoptical train surrounding the filter must be designed with the objectiveto control SMSR.

[0008] One solution to controlling SMSR used in some conventional MOEMSfilter systems is to integrate the tunable filter into the largeroptical system by locating it between two fiber pigtails; one fiberpigtail emits the optical signal to be filtered and the other fiberpigtail collects the filtered optical signal after its transmissionthrough the tunable filter. The tunable filter is oriented to beorthogonal to the axis extending between the fiber endfaces.

SUMMARY OF THE INVENTION

[0009] As optical systems are developed that allow for higher levels offunctionality in a single package, increased attention is directed tothe co-design of the tunable filter element and surrounding opticalsystem. This is especially true in systems utilizingfree-space-interconnects between the tunable filter and other opticalcomponents in the system.

[0010] One parameter that affects the SMSR of a MOEMS filter system ismode size matching between the lowest order transverse mode of thetunable filter and the mode size of the light as it is launched into thetunable filter. The mode field diameter is a measure of the radialintensity distribution of radiation. Mode field diameter is measured bythe ITU-T reference test method based on the far field scan technique.The intensity of the radiation reaching the photodiode is recorded as afunction of angle; and from these data, the mode field diameter iscalculated. According to one definition, weighted mean of the angularradial intensity distribution is used. If the mode size of the lightthat is launched into the filter is smaller or larger than the lowestorder mode of the filter, higher order modes will be excited, therebydegrading the performance of the system.

[0011] The spectral output of a Fabry-Perot filter, in general,comprises multiple spectrally distributed peaks in the filter's responseto a broadband light source. These different peaks are attributable tothe longitudinal mode orders of operation of the cavity and the cavity'stransverse spatial modes. The pattern of the peaks repeats itselfspectrally with a periodicity that is related to the separation betweenthe mirrors, termed the free spectral range. Within a given order oflongitudinal mode operation, the frequency separation between transversemodes is related to the curvature of the mirrors. Specifically, forHermite-Gaussian transverse modes the spectral separation between thelowest-order mode and any higher-order mode with mode number (n,m) aregiven by: $\begin{matrix}{{\Delta \quad v_{HOM}} = \quad {\left( {n + m + 1} \right){\arccos \left\lbrack {{{sqrt}\left( {1 - \frac{L}{r_{1}}} \right)} \cdot {{sqrt}\left( {1 - \frac{L}{r_{2}}} \right)}} \right\rbrack}{c/\left( {2\pi \quad L} \right)}}} \\{= \quad {\left( {n + m + 1} \right){{\arccos \left\lbrack {{sqrt}\left( {g_{1} + g_{2}} \right)} \right\rbrack} \cdot \frac{c}{2\pi \quad L}}}}\end{matrix}$

[0012] whereg₁=1−L/r₁ and g₂=1−L/r₂, where r1 and r1 are the radii ofcurvature of the two mirrors and L is the mirror separation.

[0013] Typically, one of the mirrors will have a known radius ofcurvature, for example, in a curved-flat cavity. Such information can bedetermined using white-light interferometery or other surfaceprofilometry. The other mirror's radius can thus be computed.

[0014] This scheme is useful in the situation where the known mirror hasa relatively small radius, and thus can be measured accurately. When thesecond mirror has a very long radius, it is difficult to measure itsradius, especially if its effective aperture is small.

[0015] The present invention is directed to a technique for determiningthe mode size of a MOEMS tunable Fabry-Perot filter by reference to acalculated value for the curvatures of the reflectors that form theFabry-Perot tunable filter cavity. Specifically, in the case of aconcentric Fabry-Perot cavity or related cavity where one of the mirrorsis relative flat, the curvature of the curved reflector is calculatedfrom the spectral response of the tunable filter.

[0016] In general, according to one aspect, the invention features aprocess for configuring a tunable MOEMS filter train. The processcomprises determining a spectral response of a MOEMS tunable filter. Aspectral separation between different order longitudinal modes, or freespectral range, is then determined for the filter, as well as transversemode spectral separation. This information is then related to a modesize of a desired mode of the tunable filter. With this information,lenses for the optical train are provisioned, and then installed so thatlight is launched into the optical filter at the desired mode size tothereby maximize the SMSR of the filter train.

[0017] In specific embodiments, the mode size of the injected opticalsignal is determined for the filter train. In the case of light beinglaunched from a single mode optical fiber, the mode size is about 8-10micrometers in diameter.

[0018] In one implementation, the spectral response of the tunablefilter can be determined by hi 15 tuning the tunable filter across alaser light source or other source that generates a spectrally narrowline. In another implementation, the filter spectral response isdetermined by injecting broadband “white” light into the filter andmeasuring the transmitted light spectrum.

[0019] According to other aspects of the preferred embodiment, the stepof determining the spectral separation comprises determining a spectralseparation between a lowest order mode and a next higher order modewithin an order of operation of the tunable filter. Using thisinformation, lenses in the optical train are selected to have beamforming characteristics that will yield the desired mode size at thetunable filter. These provisioned lenses are then installed in thefilter train.

[0020] According to another implementation, the location of the lensesin the filter train can be adjusted to achieve the desired mode size atthe tunable filter.

[0021] The above and other features of the invention including variousnovel details of construction and combinations of parts, and otheradvantages, will now be more particularly described with reference tothe accompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In the accompanying drawings, reference characters refer to thesame parts throughout the different views. The drawings are notnecessarily to scale; emphasis has instead been placed upon illustratingthe principles of the invention. Of the drawings:

[0023]FIG. 1 is a perspective view of an optical channel monitor towhich the present invention is applicable, in one example;

[0024]FIG. 2 is a schematic block diagram showing a tunable filter trainaccording to the present invention;

[0025]FIG. 3 is a process diagram illustrating the inventive tunablefilter train configuration process for mode field diameter matching; and

[0026]FIG. 4 is a spectral plot showing a lowest order mode and a nexthigher order mode within an order of operation of the tunable filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027]FIG. 1 illustrates the integration of the optical channelmonitoring system on a single, miniature optical bench 2.

[0028] Specifically, the fiber 10 is terminated on the bench 2 at amounting and alignment structure 252. This mounting and alignmentstructure 252 holds the fiber in proximity to a first collimating lens14, which is held on its own mounting and alignment structure 254. Thefirst collimating lens forms a signal beam that is transmitted throughan optional isolator 60.

[0029] After the isolator, a focusing lens 16 held on mounting andalignment structure 258 focuses the beam onto a tunable MOEMS filter 18,which is held on the filter mounting and alignment structure 259.

[0030] In one implementation, a reference signal optical train isfurther provided. Specifically, a super luminescent light emitting diode(SLED) 52 generates the broadband beam, which is focused by the secondcollimating lens 54 held on mounting and alignment structure 256. Thiscollimates the beam to pass through the etalon 56 installed on the bench2. A reference beam generated by the etalon is reflected by fold mirror58 to a first WDM filter 50 in the signal beam path. As a result, acombined beam is transmitted through the isolator 60 and the tunablefilter.

[0031] The filtered, combined beam from the filter 18 is re-collimatedby a third collimating lens 62 held on mounting and alignment structure260. This beam is then separated into the filtered reference beam andthe filtered signal beam by a second WDM filter 64. The reference signalis detected by reference photodiode 66. The filtered optical signal istransmitted through the second WDM filter 64 to the signal photodiode68.

[0032]FIG. 2 is a schematic diagram of the portion of the filter trainfor the tunable filter 18 that defines the launch criteria for theoptical signal into the tunable filter and thus, the filter's andsystem's SMSR. Specifically, the fiber 10 is preferably single mode. Itlaunches the optical signal in the form of a beam into the first lens14. This generally improves the collimation of the beam or forms a beamwaist between the first lens 14 and the second lens 16. In the preferredembodiment, the focal lengths of the first and second lenses are between1.0 and 2.0 millimeters. In a current implementation, the focal lengthof the first lens 14 is about 1100 μm and of the second lens is about1600 μm. The spacing between the first lens and the fiber endface isless than 1.0 millimeter, or presently about 500 μm. The spacing betweenthe first lens 14 and of second lens 16 is between 2 and 10 mm,presently it is about 6 mm. Finally, the spacing between the second lensand the reflecting membrane 110 of the tunable filter 18 is between 0.5and 3 mm. Presently, it is about 1 mm. In the current implementation,the membrane 110 is silicon and the curved reflector 120 is silicon orgallium phosphide.

[0033] With these parameters, the nominal magnification of the tunablefilter train, comprising lenses 14, 16, is two. Generally, themagnification should be between 1 and 5. Thus, the 10 micrometerdiameter mode size emitted from the endface 12 of the fiber 10 isconverted to a 20 micrometer beam diameter at the tunable filter 18.Generally, the mode field diameter of the lowest order mode for thefilter is between 10 and 50 micrometers.

[0034]FIG. 3 shows the process for configuring the tunable filter trainaccording to the present invention. Specifically, the mode size of theinjected signal is determined in step 310.

[0035] Specifically, this mode size is approximately 8-10 micrometers inthe current embodiment, which is the typical mode size in single modefiber for wavelengths surrounding 1,550 nm.

[0036] Also, in step 320, the spectral response of the MOEMS tunablefilter is determined. In is one implementation, a signal from a lasersource or other narrow-band signal is injected into the MOEMS tunablefilter 18, while the tunable filter is scanned across the signal. Thus,the temporal response is roughly equivalent to the filter's spectralresponse. Exemplary spectral plot is illustrated in FIG. 4. Within theillustrated order of operation, there is a lowest order mode 410, higherorder satellite modes 412, 414 that are attributed to the transversespatial modes of the FP cavity.

[0037] Next, in step 330, the spectral separation between the filtermodes is determined.

[0038] Specifically, the nanometer separation between mode 410 and mode412 in FIG. 4 is determined in the preferred embodiment, since these aretypically the highest power modes in the signal. Additionally, in thepreferred embodiment, the free spectral range of the filter isdetermined. This is the spectral separation between the different ordersof the filter operation, which correspond to the different longitudinalmodes of the filter cavity.

[0039] Next, the desired mode size is determined in step 340.Specifically, the following set of calculations are used to determinethat mode size in one embodiment in which, the mirror 120 is galliumphosphide (GaP), the membrane 110 is silicon, and the free spectralrange is 76 nanometers.

[0040] Note: All dimensions in microns.

[0041] Based on the measured HOM spacing (the odd mode-fundamental),deduce the glg2 product to calculate the radius of curvature of the MemsMembrane 110. Then, determine the mode-matched spot sized launched fromeither the Si or the GaP side. $\begin{matrix}{R_{GaP}:={- 1055}} & {{Negative}{\quad \quad}{value}\quad {of}\quad R\quad {is}\quad {for}\quad {concave}\quad {mirror}\quad \left( {{as}\quad {beam}\quad {sees}\quad {it}} \right)} \\{\lambda:={1.559034\quad µ\quad m}} & {{Fundamental}\quad {wavelength}} \\{\lambda_{next}:{1.635102\quad µ\quad m}} & {{Next}\quad {order}\quad {wavelength}} \\{\lambda_{odd}:={\lambda - {{.003388}\quad µ\quad m}}} & {\quad {{FSR}({nm})}} \\{{\Delta \quad v}:={\frac{c}{\lambda} - \frac{c}{\lambda_{next}}}} & {{{\Delta \quad v} = {{8.946 \times 10^{12}\quad {Hz}\quad {\left( {\lambda_{next} - \lambda} \right) \cdot 1000}} = {76.086\quad µ\quad m}}}\quad} \\{\underset{\_}{L}:={\frac{c}{2 \cdot {\Delta v}}}} & {\underset{\_}{L} = {16.756\quad µ\quad m}} \\{{\Delta \quad v_{HOM}}:={\frac{c}{\lambda_{odd}} - \frac{c}{\lambda_{\quad}}}} & {{\Delta \quad v_{HOM}} = {4.191 \times 10^{11\quad}{Hz}}} \\{g_{1}:=\frac{\left( {\cos \left( {\Delta \quad {v_{HOM} \cdot 2 \cdot \underset{\_}{L}}\frac{\pi}{c}} \right)} \right)^{2}}{1 + \frac{\underset{\_}{L}}{R_{GaP}}}} & \quad \\{R_{mems}:=\frac{\underset{\_}{L}}{g_{1} - 1}} & \quad \\{R_{mems}:={2949\quad µ\quad m}} & {\left( {{radius}{\quad \quad}{of}\quad {curvature}\quad {of}\quad {silicon}\quad {membrane}\quad 110} \right)\quad}\end{matrix}$

[0042] Calculate beam diameters at mirrors for a spherical resonator,then determine the optical launch condition from either Si or GaP side$\underset{\_}{{L:={16.756\quad µ\quad m}}\quad}$$\quad {\begin{matrix}{{R_{mems}:={{- 2948.9}\quad µ\quad m}}\quad} & {{\lambda:={1.559034\quad µ\quad m}}\quad} \\{{R_{GaP}:={{- 1055}\quad µ\quad m}}\quad} & \quad \\{z_{1}:=\frac{- {L\left( {R_{mems} + L} \right)}}{R_{mems} + R_{GaP} + {2 \cdot L}}} & {z_{1} = {{{- 12.374}\quad {position}\quad {of}\quad {mirror}{\quad \quad}1\quad {wrt}\quad {beam}\quad {waist}\quad {at}\quad z} = 0}} \\{z_{2}:={z_{1} + L}} & {z_{2} = {{4.382\quad {position}\quad {of}\quad {mirror}\quad 2\quad {wrt}\quad {beam}\quad {waist}\quad {at}\quad z} = 0}}\end{matrix}\begin{matrix}{{z_{0}:=\sqrt{\frac{{- L} \cdot \left( {R_{GaP} + L} \right) \cdot \left( {R_{mems} + L} \right) \cdot \left( {R_{mems} + R_{GaP} + L} \right)}{\left( {R_{mems} + R_{GaP} + {2 \cdot L}} \right)^{2}}}}\quad} & \quad \\\quad & {{{rayleigh}\quad {range}},\quad {beam}} \\\quad & {\quad {{radius}\quad {is}\quad {sqrt}\quad (2)\quad {larger}}} \\\quad & {\quad {{{t{han}}\quad {waist}\quad {here}},}\quad} \\\quad & {{2z_{0}} = {{depth}\quad {of}\quad {focus}}}\end{matrix}\begin{matrix}{{w_{0}:=\sqrt{\lambda \cdot \frac{z_{0}}{\pi}}}\quad} & {w = {7.508\quad {waist}\quad {radius}}} \\{w_{1}:={w_{0} \cdot \left\lbrack {1 + \left( \frac{z_{1}}{z_{0}} \right)^{2}} \right\rbrack^{\frac{1}{2}}}} & {w_{1} = {7.552\quad {spot}\quad {radius}\quad {at}\quad {mirror}{\quad \quad}1}} \\{w_{2}:={w_{0} \cdot \left\lbrack {1 + \left( \frac{z_{2}}{z_{0}} \right)^{2}} \right\rbrack^{\frac{1}{2}}}} & {w_{2} = {7.513\quad {spot}\quad {radius}\quad {at}\quad {mirror}{\quad \quad}2}} \\\quad & {{2 \cdot w_{0}} = 15.016}\end{matrix}}\quad$Spot  size  at  curved  mirror = 2 ⋅ w₁ = 15.105  µ  m=  spot  diameter  at  1/e²  power  Spot  size  at  flat  mirror = 2 ⋅ w₂ = 15.027  µ  m  

[0043] Calculate the optimum launch condition for mode—matching toeither side. Calculate the required spot size in air since we canmeasure it directly.

[0044] Launching from the GaP Mirror $\begin{matrix}{n:=3.052} & {L:=200} \\{{r_{c}:=R_{GaP}}\quad} & {W_{c}:=W_{1}}\end{matrix}$$q_{2} = \left\lbrack \left( {\frac{1}{r_{c}} - \frac{i \cdot \frac{\lambda}{n}}{\pi \cdot w_{c}^{2}}} \right)^{- 1} \right\rbrack$q₂ := −105.021 + 315.86i $\begin{matrix}{q_{0{im}}:={{Im}\left( \frac{q_{2} - L}{n} \right)}} & {q_{0{re}}:={{Re}\left( \frac{q_{2} - L}{n} \right)}} & {q_{0{im}} = 103.493} \\{q_{0}:={q_{0{re}} + {i \cdot_{0{im}}}}} & \quad & {q_{0{re}} = {- 99.941}}\end{matrix}$

[0045] The radius of curvature and spot entering the GaP Mirror are$\begin{matrix}{R_{in}:=\frac{1}{{Re}\left( \frac{1}{q_{0}} \right)}} & {R_{in} = {- 207.112}} \\{w_{in}:=\sqrt{\frac{- \lambda}{\pi} \cdot \frac{1}{{{Im}\left( \frac{1}{q_{0}} \right)}\quad}}} & {{2w_{in}} = 19.925} \\\quad & {w_{in} = 9.963}\end{matrix}$

[0046] Therefore, the spot size at the waist in air is $\begin{matrix}{w_{0}:=\frac{w_{in}}{{\sqrt{1 + \left( \frac{\pi \cdot w_{in}^{2}}{\lambda \cdot R_{in}} \right)}}^{2}}} & {{2w_{0}} = 14.333} \\\quad & {w_{0} = 7.167}\end{matrix}$

[0047] Launching from Si Membrane side $\begin{matrix}{n:=3.4} & {L:=7} & \quad \\{r_{c}:=R_{mems}} & {W_{c}:=W_{2}} & \quad \\{{q2}:=\left\lbrack \left( {\frac{1}{r_{c}} - \frac{i \cdot \frac{\lambda}{n}}{\pi \cdot w_{c}^{2}}} \right)^{- 1} \right\rbrack} & \quad & \quad \\{{q2} = {{- 49.869} + {380.227i}}} & \quad & \quad \\{q_{0{im}} = {:={{Im}\left( \frac{q_{2} - L}{n} \right)}}} & {q_{0{re}}:={{Re}\left( \frac{q_{2} - 1}{n} \right)}} & {q_{0{im}} = 111.831} \\{q_{0:} = {q_{0{re}} + {i \cdot q_{0{im}}}}} & \quad & {q_{0{re}} = {- 16.276}}\end{matrix}$

[0048] The radius of curvature and spot entering the Si Membrane are$\begin{matrix}{R_{in}:=\frac{1}{{Re}\left( \frac{1}{q_{0}} \right)}} & {R_{in}:={- 764.433}} \\{w_{in}:=\sqrt{\frac{- \lambda}{\pi} \cdot \frac{1}{{Im}\left( \frac{1}{q_{0}} \right)}}} & {{2w_{in}} = 15.065} \\\quad & {w_{in} = 7.532}\end{matrix}$

[0049] Therefore, the spot size at the waist in air is $\begin{matrix}{w_{0}:=\frac{w_{in}}{\sqrt{1 + \left( \frac{\pi \cdot w_{in}^{2}}{\lambda \cdot R_{in}} \right)^{2}}}} & {{2w_{0}} = 14.899} \\\quad & {w_{0} = 7.45}\end{matrix}$

[0050] Once the desired mode size for the tunable filter 18 isdetermined, then the lenses of the filter train are selected and theirposition is determined in step 350.

[0051] Specifically, according to the illustrated embodiment, there aretwo lenses in the filter train: the first lens 14 and the second lens16. These yield an effective magnification between the mode size at thefiber endface 12 and the tunable filter 18.

[0052] Lenses of established curvatures can be used. The positioning ofthe lenses in the filter train between the fiber endface and the tunablefilter is adjusted to yield the preferred mode size at the tunablefilter.

[0053] Finally, in step 360, the lenses are installed in the filtertrain having the selected curvatures and locations between the fiberendface 12 and the tunable filter 18.

[0054] A further extension of above described techniques is to measureastigmatism in the mirrors. Mirror astigmatism is manifested in thespectral plot of the filtering function by peak splitting in the higherorder modes. Measurement of the spectral distance between thesesub-peaks is related to the astigmatism in the mirror, or specificallythe two radii of curvatures.

[0055] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A process for configuring a tunable MOEMS filtertrain, the process comprising: determining a spectral response of aMOEMS tunable filter; determining a spectral separation betweendifferent modes in the spectral response of the tunable filter;determining a mode size of a desired mode of the tunable filter from thespectral separation; and selecting and installing an optical componentin response to the determined mode size into an optical train of thetunable filter to launch light into the tunable filter.
 2. A process asclaimed in claim 1, further comprising determining a mode size of anoptical signal injected into the filter train.
 3. A process as claimedin claim 1, wherein a mode size of an optical signal injected into thefilter train is about 10 micrometers in diameter.
 4. A process asclaimed in claim 1, further comprising injecting an optical signal intothe filter train directly from a single mode optical fiber.
 5. A processas claimed in claim 1, wherein the step of determining the spectralresponse of the tunable filter comprises scanning the tunable filteracross a laser light source.
 6. A process as claimed in claim 1, whereinthe step of determining the spectral response of the tunable filtercomprises scanning the tunable filter across a spectrally narrow line.7. A process as claimed in claim 1, wherein the step of determining thespectral separation between the different modes in the spectral responseof the tunable filter comprises determining the spectral separationbetween a lowest order mode and a next higher order mode within an orderof operation of the tunable filter.
 8. A process as claimed in claim 1,wherein the step of determining the mode size comprises determining themode size of a lowest order mode of the tunable filter.
 9. A process asclaimed in claim 1, wherein the step of selecting and installing theoptical component comprises selecting a lens having beam formingcharacteristics that will yield the determined mode size at the tunablefilter.
 10. A process as claimed in claim 1, wherein the step ofselecting and installing the optical component comprises determining thebeam forming characteristics of the optical component and determining aposition for the optical component that will yield the determined modesize at the tunable filter.
 11. A process as claimed in claim 1, whereinthe step of selecting and installing the optical component comprisesdetermining the beam forming characteristics of the optical componentand installing the optical component to provide a mode field diameter ofbetween 10 and 50 micrometers at the tunable filter.
 12. A tunable MOEMSfilter train, comprising: a MOEMS tunable filter having a spectralresponse, in which a spectral separation between different modes in thespectral response has been measured and a mode size of a desired mode ofthe tunable filter determined; and an optical component that launches aninput signal into the tunable filter, the optical component beingselected and/or placed so that the input signal has the determined modesize at the tunable filter.
 13. A filter train as claimed in claim 12,wherein the optical component comprises a lens.
 14. A filter train asclaimed in claim 12, wherein the determined mode size is between and 50micrometers.